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| <title>Using the GNU Compiler Collection (GCC): Inline</title> |
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| Next: <a href="Volatiles.html#Volatiles" accesskey="n" rel="next">Volatiles</a>, Previous: <a href="Alignment.html#Alignment" accesskey="p" rel="prev">Alignment</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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| <hr> |
| <a name="An-Inline-Function-is-As-Fast-As-a-Macro"></a> |
| <h3 class="section">6.39 An Inline Function is As Fast As a Macro</h3> |
| <a name="index-inline-functions"></a> |
| <a name="index-integrating-function-code"></a> |
| <a name="index-open-coding"></a> |
| <a name="index-macros_002c-inline-alternative"></a> |
| |
| <p>By declaring a function inline, you can direct GCC to make |
| calls to that function faster. One way GCC can achieve this is to |
| integrate that function’s code into the code for its callers. This |
| makes execution faster by eliminating the function-call overhead; in |
| addition, if any of the actual argument values are constant, their |
| known values may permit simplifications at compile time so that not |
| all of the inline function’s code needs to be included. The effect on |
| code size is less predictable; object code may be larger or smaller |
| with function inlining, depending on the particular case. You can |
| also direct GCC to try to integrate all “simple enough” functions |
| into their callers with the option <samp>-finline-functions</samp>. |
| </p> |
| <p>GCC implements three different semantics of declaring a function |
| inline. One is available with <samp>-std=gnu89</samp> or |
| <samp>-fgnu89-inline</samp> or when <code>gnu_inline</code> attribute is present |
| on all inline declarations, another when |
| <samp>-std=c99</samp>, <samp>-std=c11</samp>, |
| <samp>-std=gnu99</samp> or <samp>-std=gnu11</samp> |
| (without <samp>-fgnu89-inline</samp>), and the third |
| is used when compiling C++. |
| </p> |
| <p>To declare a function inline, use the <code>inline</code> keyword in its |
| declaration, like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">static inline int |
| inc (int *a) |
| { |
| return (*a)++; |
| } |
| </pre></div> |
| |
| <p>If you are writing a header file to be included in ISO C90 programs, write |
| <code>__inline__</code> instead of <code>inline</code>. See <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>. |
| </p> |
| <p>The three types of inlining behave similarly in two important cases: |
| when the <code>inline</code> keyword is used on a <code>static</code> function, |
| like the example above, and when a function is first declared without |
| using the <code>inline</code> keyword and then is defined with |
| <code>inline</code>, like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">extern int inc (int *a); |
| inline int |
| inc (int *a) |
| { |
| return (*a)++; |
| } |
| </pre></div> |
| |
| <p>In both of these common cases, the program behaves the same as if you |
| had not used the <code>inline</code> keyword, except for its speed. |
| </p> |
| <a name="index-inline-functions_002c-omission-of"></a> |
| <a name="index-fkeep_002dinline_002dfunctions-1"></a> |
| <p>When a function is both inline and <code>static</code>, if all calls to the |
| function are integrated into the caller, and the function’s address is |
| never used, then the function’s own assembler code is never referenced. |
| In this case, GCC does not actually output assembler code for the |
| function, unless you specify the option <samp>-fkeep-inline-functions</samp>. |
| Some calls cannot be integrated for various reasons (in particular, |
| calls that precede the function’s definition cannot be integrated, and |
| neither can recursive calls within the definition). If there is a |
| nonintegrated call, then the function is compiled to assembler code as |
| usual. The function must also be compiled as usual if the program |
| refers to its address, because that can’t be inlined. |
| </p> |
| <a name="index-Winline-1"></a> |
| <p>Note that certain usages in a function definition can make it unsuitable |
| for inline substitution. Among these usages are: variadic functions, use of |
| <code>alloca</code>, use of variable-length data types (see <a href="Variable-Length.html#Variable-Length">Variable Length</a>), |
| use of computed goto (see <a href="Labels-as-Values.html#Labels-as-Values">Labels as Values</a>), use of nonlocal goto, |
| and nested functions (see <a href="Nested-Functions.html#Nested-Functions">Nested Functions</a>). Using <samp>-Winline</samp> |
| warns when a function marked <code>inline</code> could not be substituted, |
| and gives the reason for the failure. |
| </p> |
| <a name="index-automatic-inline-for-C_002b_002b-member-fns"></a> |
| <a name="index-inline-automatic-for-C_002b_002b-member-fns"></a> |
| <a name="index-member-fns_002c-automatically-inline"></a> |
| <a name="index-C_002b_002b-member-fns_002c-automatically-inline"></a> |
| <a name="index-fno_002ddefault_002dinline"></a> |
| <p>As required by ISO C++, GCC considers member functions defined within |
| the body of a class to be marked inline even if they are |
| not explicitly declared with the <code>inline</code> keyword. You can |
| override this with <samp>-fno-default-inline</samp>; see <a href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options">Options Controlling C++ Dialect</a>. |
| </p> |
| <p>GCC does not inline any functions when not optimizing unless you specify |
| the ‘<samp>always_inline</samp>’ attribute for the function, like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">/* <span class="roman">Prototype.</span> */ |
| inline void foo (const char) __attribute__((always_inline)); |
| </pre></div> |
| |
| <p>The remainder of this section is specific to GNU C90 inlining. |
| </p> |
| <a name="index-non_002dstatic-inline-function"></a> |
| <p>When an inline function is not <code>static</code>, then the compiler must assume |
| that there may be calls from other source files; since a global symbol can |
| be defined only once in any program, the function must not be defined in |
| the other source files, so the calls therein cannot be integrated. |
| Therefore, a non-<code>static</code> inline function is always compiled on its |
| own in the usual fashion. |
| </p> |
| <p>If you specify both <code>inline</code> and <code>extern</code> in the function |
| definition, then the definition is used only for inlining. In no case |
| is the function compiled on its own, not even if you refer to its |
| address explicitly. Such an address becomes an external reference, as |
| if you had only declared the function, and had not defined it. |
| </p> |
| <p>This combination of <code>inline</code> and <code>extern</code> has almost the |
| effect of a macro. The way to use it is to put a function definition in |
| a header file with these keywords, and put another copy of the |
| definition (lacking <code>inline</code> and <code>extern</code>) in a library file. |
| The definition in the header file causes most calls to the function |
| to be inlined. If any uses of the function remain, they refer to |
| the single copy in the library. |
| </p> |
| <hr> |
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